Fuel cell systems in which hydrogen is supplied to an anode side as a fuel gas and air is supplied to a cathode side as an oxidizing gas, in between of which is provided a membrane electrode assembly (MEA) comprised of stacked solid state electrolyte membrane and catalyst layers, are well known. In such a fuel cell operation system, electric power is generated by reaction of hydrogen and oxygen in the air within the cell which includes the MEA, and water, which is a reaction product, is discharged from the cathode side. In order to adjust the amount of air to be supplied to the cathode side, a back pressure valve or a control valve, known as a pressure regulating valve, is provided at the cathode side outlet of the fuel cell.
For example, Japanese Patent Publication JP 2003-180006 A discloses a regenerative braking system for use in fuel cell vehicles in which an expensive battery pack is not used, wherein a back pressure valve is provided between the cathode side outlet of the fuel cell and an atmospheric open end, and a bypass valve which is a three-way valve for bypassing to the atmospheric open end is provided between the cathode and a compressor which supplies compressed air. This publication further describes that during regenerative braking the back pressure valve is closed and the bypass valve is adjusted to regulate the compressor load of the air compressor, thereby generating an artificial loss to regulate regenerative torque.
The bypass valve described in the above-described JP 2003-180006 A is used to perform regenerative braking in a fuel cell which does not use a battery pack, which is a special case, whereas in other cases, a bypass valve is used to supply a diluent gas for diluting hydrogen gas for exhaustion to the atmosphere. More specifically, because the MEA is composed of stacked membranes, hydrogen may partially leak from the anode side to the cathode side, and this leaking hydrogen and unreacted hydrogen present at the anode side are diluted with air and discharged. For this purpose, a bypass valve divides air also serving as an oxidizing gas into a gas for reaction within a cell and a gas for dilution.
As described above, in a fuel operation system which uses hydrogen as a fuel gas, in addition to hydrogen gas supply and air supply, the generation of power, discharge of water as a reaction product, dilution of exhaust gas, and the like are regulated by the operation of several valves.
For such valves in a fuel cell operation system, on the basis of power generation and the like required of the fuel cell, detection results obtained by a pressure sensor and the like provided in a gas flow path are used to calculate valve open amount instructions to perform control.
However, the pressure sensor and the like may cease functioning at low temperatures. In such cases, it will be impossible to calculate valve open amount instructions.
An advantage of the present invention is to provide a fuel cell operation system which can calculate a valve open amount instruction without using a pressure sensor or the like, and a valve open amount calculation method in the fuel cell operation system.